The Life Cycle of Giant Molecular Clouds Charlotte Christensen

Observational Constraints on The Life Cycle of Giant Molecular Clouds in Milky Way-like Galaxies Charlotte Christensen

Coming up Physical Background Lifecycle Formation Core Formation Protostar Formation Star Formation Dispersal Nagging Questions

Meet the Molecules

HII

HI

H2H2H2H2

12 CO

Meet the Molecules 13 CO

Meet the Molecules NH 3

3 Phase Interstellar Media Hot Ionized Medium Warm Neutral/Ionized Medium Cold Neutral Medium

3 Phase Interstellar Media Hot Ionized Medium HII T  K   cm -3 Warm Neutral/Ionized Medium Cold Neutral Medium Haffner et al, 2003

3 Phase Interstellar Media Hot Ionized Media Warm Neutral/Ionized Media HII & HI T  ,000K   0.01 cm -3 Cold Neutral Media MW 21cm radiation Dickey & Lockman, 1990

3 Phase Interstellar Media Hot Ionized Media Warm Neutral/Ionized Media Cold Neutral Media HI & H 2 T  K   cm -3 Dame et al, 2001 MW CO emission

Molecular Hydrogen Clouds Self-gravitating (rather than diffuse) H 2, molecules, and dust grains % of the gas mass Occupy > 1% of the volume Site of star formation Eagle Nebula HST

Size Scales Mass (M O )Size (pc)  (cm -3 ) Superclouds / GMAs Giant Molecular Clouds Molecular Clouds Bok Globules Cores

Size Scales Mass (M O )Size (pc)  (cm -3 ) Superclouds / GMAs Giant Molecular Clouds Molecular Clouds Bok Globules Cores

Some Timescales Crossing Time Time for a sound wave to propagate through  c =  10 Myr Dynamical Time Time for a particle to free fall to center  dyn = G  -1/2  2 Myr “Dynamic” vs “Quasi-Static” Evolution

Support Assume Equilibrium Virial Theorem 2 T + W = 0 Kinetic Energy Potential Energy Jeans Mass:

Support Assume Equilibrium Outside Pressure 2(T - T 0 ) + W = 0 Potential Energy KE from External Pressure Kinetic Energy

Support Assume Equilibrium Turbulence vs Thermal KE 2(T  + T P - T 0 ) + W = 0 Potential Energy KE from External Pressure Thermal KE Turbulent KE

Support Assume Equilibrium Magnetic Field 2(T  + T P - T 0 ) + W + B = 0 Potential Energy KE from External Pressure Thermal KE Turbulent KE Mag. Enegry

Support Assume Equilibrium Magnetic Field 2(T  + T P - T 0 ) + W + B = 0 Potential Energy KE from External Pressure Thermal KE Turbulent KE Mag. Enegry

Turbulent Support -- Source Internal Stellar Winds Bipolar Outflows HII External Density Waves Differential Rotation Supernovae Winds from Massive Stars

Turbulent Support -- Decay Close to a Kolmogrov Spectrum Cascade down to lower energies Large eddies form small eddies Small eddies dissipated through friction Timescale:  1 Myr

Magnetic Field Support -- Source Galactic Dynamo Seed Magnetic Field Differential Rotation Convection Throughout MW Seen in polarization and Zeeman splitting MPIfR Bonn NGC 6946

Magnetic Field Support -- Decay Ambipolar Diffusion -- Decoupling of charged and neutral particles Timescale: 10 Myr Depends on: Density Magnetic Flux Ionization Fraction

Life Cycle Cloud Formation Cloud Core Formation Protostar Collapse Stars Form Cloud Dispersal

Life Cycle Cloud Formation Cloud Core Formation Protostar Collapse Stars Form Cloud Dispersal

Theories Collisional build up of molecular clouds Growth time  collisional time Quiescent growth of ambient H 2 Gravitational/magnetic instability Shock compression Spiral Arms Supernovae From HI of H 2 ?

w/ CO all HI Correlation with HI Filaments of HI around all GMCs Engargiola et al, 2003 M33 Density

Correlation with Spiral Arms M33  60% of H 2 in spiral arms Grand design spirals: > 90% (Nieten et al. 2006, Garcia-Burillo et al 1993) Rosolowsky et al, 2007

Age Limits  = Myr Collisional build up of molecular clouds  = 2000 Myr Quiescent growth of ambient H 2  H2 = 0.3 M O pc 2  = 100 Myr Engargiola et al, 2003 M33

Shocks Observation of a shocked GMA Tosaki, C 13 C M31

GMC Formation -- Conclusions Formed primarily from either HI or H 2 Compressed to self- gravitating clouds in spiral arms

Life Cycle Cloud Formation Cloud Core Formation Protostar Collapse Stars Form Cloud Dispersal

Cloud Core Formation GMC is supported by: Magnetic flux Turbulence Support is removed either Slowly by Ambipolar diffusion Fast by decay of turbulence and turbulence amplified diffusion Cores (regions 2-4 times ambient density) form at  10% efficiency Lagoon Nebula

Initial Conditions Cloud envelope is In non-equilibrium Magnetically subcritical (Cortes et al, 2005) Very inhomogenous Carina, HST

Observations of Cores Myers & Fuller, 1991

Observations of Cores Cores are: Non-isotropic More prolate than oblate Not necessarily aligned with the magnetic field (Glenn 1999) Prolate Oblate

Ratio of Clouds without Stars One last test of timescale: N NS /N T =  NS /  T Cloud Formation Cloud Core Formation Protostar Collapse Stars Form Cloud Dispersal

Ratio of Clouds without Stars Very few MW GMCs without SF 25% of GMCs in other galaxies have no associate HII regions (Blitz, 2006) Engargiola, et al 2003 M33 -- Distance between GMC and HII

Ratio of Clouds without Stars N NS /N T =  NS /  T  1/4 Dynamic Collapse Cloud Formation Cloud Core Formation Protostar Collapse Stars Form Cloud Dispersal

Life Cycle Cloud Formation Cloud Core Formation Protostar Collapse Stars Form Cloud Dispersal

Core Collapse to Protostar Overdensties collapse Collapse regulated by Turbulence Magnetic Field Fragmentation Protostar formation when core becomes opaque

Core Sizes &Densities Radius (pc) Lee et al, 1999 Enoch et al, 2008 Log Density

Protostar Formation Size

Magnetic Support Cores are (probably) supercritical, i.e. not supported by the magnetic field M/  B = c  G -1/2 c   0.12 Crutcher, 1999 Critical

Turbulence Cores are turbulent Motions are Supersonic Turbulence from shocks or MHD waves Myers & Khersonsky, 1994

MHD Turbulence Dependent on Ionization Decays by *** Decay rate is still comparable to non- magnetic turbulence Speeds close to Alfven speed

Time Scales We have flow of material onto magnetically- unsupported cores Larger, more massive cores collapse to protostars How fast does this happen?

Time Scales -- Spiral Arm Offset

Tosaki, 2002 M51 13 CO 12 CO HH

Time Scales -- Spiral Arm Offset Difference between peaks  10 Myr Long delay of SF OR staggered SF Tosaki, 2002

Time Scales -- Statistcs Ratio of clouds without protostars: N NSC /N C =  NSC /  C Cloud Formation Cloud Core Formation Protostar Collapse Stars Form Cloud Dispersal

Time Scales -- Statistics Optically Selected MW Cores: N NSC /N C = 306/400 (Lee & Myers, 1999) Perseus, Serpens, & Ophiuchus: N NSC /N C = 108/200 (Enoch et al, 2008) 25% - 50% of core life before SF (Enoch et al, 2008)

Time Scales -- Statistics Lifetime of a protostar  x 10 5 Myr Lifetime of a core  x 10 6 Myr Cloud Formation Cloud Core Formation Protostar Collapse Stars Form Cloud Dispersal 0.5 Myr

Life Cycle Cloud Formation Cloud Core Formation Protostar Collapse Stars Form Cloud Dispersal

Stars Form Powered by gravitational energy Envelopes of accreting material T Tauri Stars Trifid, HST

Size Hatchel & Fullerl, 2008 Younger Protostar Older Protostar Starless Perseus Cores

Time Scale T Tauri Problem Most stars form within 3 Myr Palla & Stahler, 2000

Location Huff & Stahler, 2006

Time Scale Star formation lasts  Myr Clouds gone after Myr Cloud Formation Cloud Core Formation Protostar Collapse Stars Form Cloud Dispersal Myr

Lifecycle Cloud Formation Cloud Core Formation Protostar Collapse Stars Form Cloud Dispersal

Clouds Dispersing Leisawitz, 1989

Proximity to New Stars Star clusters older than 10 Myr have no associated clouds Leisawitz, 1989

Cascading SF Dispersing clouds may spark SF elsewhere Hartmann M51, HST

Putting it all Together Cloud Core Formation Protostar Collapse Stars Form Cloud Dispersal Cloud Formation Cascading SF Myr

Nagging Questions Do clouds form from HI of H 2 ? How long before cores form? What effect does the magnetic field have on turbulence?

Thanks Tom Quinn, Fabio Governato, Julianne Dalcanton, Andrew Connely, Bruce Hevly Adrienne and David for making me dinner Everybody who came to my practice talk

Gas In-fall Onto Cores Lee, 2001

Alignment

MHD Turbulence Padoan, 2004

Core Densities Enoch, 2008

Location Huff & Stahler, 2006

More Dispersal Jorgensen, 2007